What Are Coils in a Car Engine?

Ignition coils are the components that transform a car’s 12-volt battery power into the tens of thousands of volts needed to jump the spark plug gap and ignite the air-fuel mixture; without them, a gasoline engine won’t run. They act as high-voltage step-up devices, controlled by the engine computer to deliver precisely timed sparks under all driving conditions.

What Ignition Coils Do

At their core, ignition coils are compact transformers. The engine control unit (ECU) charges a coil’s primary winding, building a magnetic field; when the current is cut, the field collapses and induces a high-voltage pulse in the secondary winding. That pulse travels to a spark plug, arcs across its gap, and ignites the mixture. Modern coils must perform reliably at high temperatures and under rising cylinder pressures found in turbocharged engines, which increases spark demand.

How They Work, Electrically

Unlike a household transformer running on continuous AC, a coil in a car is an inductive energy device: the ECU controls “dwell” time to charge the coil, then abruptly stops the current to generate the spark. Primary current typically reaches several amps; the resulting secondary voltage can exceed 20,000–40,000 volts, with spark energy measured in millijoules. Some “smart” coils integrate their own driver transistors, thermal protection, and diagnostics, improving accuracy and reliability.

Where They Sit in Today’s Engines

Decades ago, a single coil fed a distributor that routed spark to each cylinder. Today, most gasoline engines use one coil per cylinder mounted directly on the spark plug—called coil-on-plug (COP)—to shorten the path and improve control. Some engines use paired coils in “waste-spark” systems. Diesel engines are different: they ignite fuel by compression and generally have no ignition coils, relying instead on glow plugs for cold starts. Battery-electric vehicles have no ignition at all; hybrids with gasoline engines still use coils.

The configurations below show how coil placement and control have evolved to balance cost, performance, and packaging.

• Coil-on-plug (COP): A dedicated “pencil” coil sits atop each spark plug for precise timing and strong spark.
• Coil packs (waste-spark): One coil serves two cylinders, firing both plugs simultaneously—one spark does work, the other is “wasted.”
• Single coil with distributor (older designs): One coil feeds all plugs via a rotating distributor cap and rotor.
• Smart coils: Coils with built-in drivers and diagnostics, reducing ECU heat and simplifying wiring.

Each layout has trade-offs: COP offers the best control and efficiency, while packs and distributors can be cheaper but are more prone to crossfire and energy losses.

Symptoms of a Failing Ignition Coil

Because coils are central to spark delivery, their failures tend to show up as misfires and drivability issues, sometimes intermittently as heat builds. Recognizing the signs early can prevent catalytic converter damage and poor fuel economy.

• Rough idle, stumbling, or hesitation under load or during acceleration.
• Check-engine light with misfire codes (P0300 for random/multiple, P0301–P0308 for specific cylinders) or coil circuit faults (P0351–P0358).
• Hard starts, especially when hot, or sudden engine shutoff followed by restart after cooling.
• Noticeable drop in fuel economy and power; sulfur smell from an overheated catalytic converter after prolonged misfire.
• Audible ticking/arching, visible arcing at night, or oil in spark plug tubes causing misfire.

Because many issues can mimic coil failure (such as worn plugs or vacuum leaks), proper diagnosis is important before replacing parts.

Why Coils Fail—and How to Prevent It

Heat, vibration, and excessive voltage demand are the main enemies. Modern turbocharged and direct-injected engines run hotter, and wider plug gaps or incorrect plugs make coils work harder.

• Age and heat cycling degrade insulation and internal windings.
• Worn or incorrectly gapped spark plugs raise required voltage, stressing the coil.
• Oil or coolant intrusion into plug wells (often from valve cover gasket leaks) causes carbon tracking and arcing.
• Moisture and corrosion in connectors or boots lead to misfires.
• Inferior or mismatched aftermarket parts can alter dwell/drive behavior and reduce lifespan.

Addressing root causes—especially leaks and incorrect plug gaps—prevents repeat failures and protects the catalytic converter.

Maintenance Tips That Help Coils Last

Routine service and good installation practices go a long way to keeping the ignition system healthy.

• Replace spark plugs on schedule (intervals vary by plug type and engine) and use OEM-spec plugs with the correct gap.
• Fix oil leaks into plug tubes promptly; clean and dry wells before fitting coils.
• Use a small amount of dielectric grease inside the coil boot to seal out moisture; avoid greasing electrical terminals.
• Torque spark plugs correctly; overtightening can damage threads or crack insulators.
• If one coil fails on a high-mileage engine, replace just the bad coil unless others show issues; keeping spares for known-problem engines can reduce downtime.

These steps minimize stress on the coils, reduce misfires, and maintain fuel economy and emissions performance.

Diagnosis and Replacement

Professional diagnosis combines scan data, visual inspection, and sometimes oscilloscope testing. DIYers can narrow issues safely with basic checks, but take precautions around high voltage.

1. Pull codes and look at misfire counters by cylinder; note if misfires rise with heat or load.
2. Inspect plugs, coil boots, and plug wells for oil, moisture, or carbon tracking; correct any leaks.
3. Swap the suspected coil with another cylinder and see if the misfire follows; if it does, the coil is likely faulty.
4. Check plug condition and gap; replace worn or incorrect plugs before condemning a coil.
5. For deeper analysis, use an oscilloscope to observe primary/secondary waveforms and burn time; resistance checks alone are often inconclusive.
6. When replacing, seat the coil fully, connect firmly, and clear codes; verify no misfires under load.

If misfires persist after coil and plug checks, consider injector issues, compression problems, or intake leaks as next steps.

Modern Trends and Tech

Ignition systems continue to evolve as engines demand more precise combustion control and higher reliability, especially under boost and with strict emissions standards.

• Smart coils with integrated drivers reduce ECU heat and enable better diagnostics.
• Ion-sensing systems (used by some manufacturers) monitor combustion via the plug/coils to detect knock and misfires in real time.
• Multi-spark strategies at low rpm improve idle stability; at higher rpm, a single strong spark is favored.
• Turbocharged, high-compression engines and lean-burn strategies push coils to deliver stronger, more consistent sparks.
• Hybrids still rely on robust coils for frequent start-stop cycles; EVs eliminate ignition systems entirely.

The direction is clear: tighter integration, smarter control, and higher energy delivery tailored to demanding combustion environments.

Safety Notes

Ignition coils produce very high voltage with potentially painful shocks and risks for people with implanted medical devices. Exercise caution whenever the engine is running or the key is on.

• Do not touch coils, plugs, or leads with the engine running; disable the ignition before handling.
• Use insulated tools and wear eye protection; keep flammable vapors away.
• Disconnect the battery when servicing coils on systems that might energize with key-on events.

A few simple precautions make ignition work much safer for DIYers and professionals alike.

Summary

Ignition coils are the high-voltage heart of a gasoline engine’s spark system, converting 12 volts into the powerful pulses that fire each spark plug. Most modern engines use coil-on-plug designs for precise control, while older setups rely on coil packs or a single distributor-fed coil. Recognizing symptoms of failure, addressing root causes like worn plugs or oil leaks, and following safe diagnostic practices can keep engines running smoothly and efficiently.

What happens when a coil goes bad on a car?

When an ignition coil goes bad, your car’s engine will likely misfire, causing rough idling, reduced power, and poor acceleration, along with a lit check engine light. You might also experience difficulty starting, fuel economy issues, and backfiring sounds from the exhaust. Driving with a bad coil can lead to costly damage to the catalytic converter from unburned fuel and can even cause engine damage.
 
Common Symptoms

• Engine Misfire: The engine may shake, sputter, or run roughly as one or more cylinders fail to ignite the fuel-air mixture. 
• Check Engine Light: A diagnostic trouble code is generated, and the check engine light will illuminate, often indicating a misfire. 
• Reduced Power & Poor Acceleration: The engine may struggle to pick up speed, feeling sluggish and less responsive. 
• Difficulty Starting: The engine might crank but struggle to start or may require repeated attempts to get it running. 
• Backfiring: You may hear loud banging or popping noises from the tailpipe due to unburned fuel exploding in the exhaust system. 
• Poor Fuel Economy: The engine compensates for the misfiring cylinder by burning more fuel, leading to worse gas mileage. 

What Happens to the Engine

• Catalytic Converter Damage: Opens in new tabUnburned fuel from the misfiring cylinder can enter the catalytic converter, causing it to overheat, get damaged, and potentially destroy itself. 
• Engine Damage: Opens in new tabRaw fuel can wash oil off the cylinder walls, diluting the engine oil in the oil pan, which can lead to bearing damage and other internal component failures. 

What to Do

1. Don’t Ignore the Signs: Address the issue as soon as you notice symptoms to prevent further damage. 
2. Get a Diagnostic Scan: A professional mechanic can use a diagnostic tool to identify the specific coil that is failing by reading the trouble codes. 
3. Avoid Driving: If possible, avoid driving the vehicle, especially at high speeds or when the check engine light is flashing. 
4. Have it Inspected: Seek professional help from a qualified technician to have the coil replaced and any potential damage assessed. 

How much does it cost to replace an engine coil?

Replacing an engine ignition coil typically costs between $150 and $600 for a single coil, with the average falling around $230 to $330, though costs vary significantly based on your vehicle’s make and model. This total includes both the price of the coil itself, which can range from $50 to $300 or more, and labor costs, usually billed at $50 to $150 per hour. More complex or luxury vehicle parts and labor at a dealership will increase the overall price. 
Cost Breakdown

• Parts: A single ignition coil can cost anywhere from $50 to over $300, depending on your car’s make and model. 
• Labor: Labor charges for replacement are typically $50 to $150 per hour, with the total time ranging from 0.5 to 2 hours depending on the vehicle and coil location. 

Factors Influencing the Cost

• Vehicle Type: Imported vehicles, especially Japanese models, often have higher part and labor costs than domestic vehicles. 
• Coil Type: Newer coil-on-plug (COP) systems can be more expensive to replace than older, conventional ignition systems. 
• Location: Dealerships generally charge higher rates for both parts and labor than independent repair shops. 
• Coil Location: Coils located in harder-to-reach areas or requiring other parts to be removed will increase labor time and cost. 
• OEM vs. Aftermarket: OEM (original equipment manufacturer) parts tend to be more expensive but are often recommended for their longevity. 

When to Seek Professional Help
If you are unsure about the cause of your engine problem, it’s best to have a skilled mechanic diagnose the issue, according to Synchrony. A qualified professional can accurately identify the faulty coil and perform the replacement, saving you potential further damage and ensuring the job is done correctly.

What do engine coils do?

Engine coils (specifically ignition coils) transform the battery’s low-voltage electricity into a high-voltage spark needed to ignite the air-fuel mixture in the engine’s cylinders. They act as transformers, using electromagnetic induction to boost the 12-volt current to thousands of volts. This high-voltage pulse is sent to the spark plugs, creating the spark that initiates combustion and powers the engine.
 
This video explains how ignition coils work and their function in a car’s ignition system: 1mNational MagLabYouTube · Oct 3, 2019
Here’s a breakdown of how they work:

1. Voltage Conversion: Opens in new tabThe ignition coil receives a low-voltage current (around 12 volts) from the car’s battery. 
2. Magnetic Field Generation: Opens in new tabThis low-voltage current passes through a primary coil, creating a magnetic field around an iron core. 
3. High Voltage Induction: Opens in new tabWhen the current is interrupted, the magnetic field rapidly collapses, inducing a much higher voltage in a secondary coil (which has more wire windings than the primary coil). 
4. Spark Plug Ignition: Opens in new tabThis high-voltage electricity is then sent to the spark plugs. 
5. Combustion: Opens in new tabThe spark plugs produce a powerful electric spark that ignites the air-fuel mixture in the engine’s cylinders, causing the engine to run. 

Without a functioning ignition coil, the spark plugs wouldn’t receive the high-voltage energy they need to create a strong enough spark, leading to engine sputtering, misfires, or failure to start.

How to tell if a motor coil is bad?

6 Signs of a Faulty Ignition Coil

1. Signs of a Faulty Ignition Coil: Check Engine Light.
2. The Engine Starts Misfiring.
3. The Engine Won’t Start Normally.
4. The Engine Guzzles Gas.
5. The Car Doesn’t Perform Well.
6. The Car Backfires.